SEOUL, South Korea — With diplomacy moving apace to end North Korea’s nuclear weapons program, Northeast Asia is bracing for something few had thought likely just months ago: a reduction or withdrawal of American troops from South Korea.

These forces have been the bedrock of the 65-year-old alliance between Seoul and Washington since the 1950-53 Korean War, serving as a bulwark against North Korean aggression and preserving a shaky peace that allowed South Korea to build its economy into a global powerhouse.

Now their presence is being questioned by President Trump, who is skeptical of maintaining a costly American military presence overseas, and by North Korea’s leader, Kim Jong-un, who called last week for a peace treaty formally ending the Korean War.

As Mr. Trump and Mr. Kim prepare to meet within the next few weeks to discuss peace and an end to North Korea’s nuclear program, Mr. Trump has ordered the Pentagon to prepare options for reducing the number of American troops in the South, according to several people briefed on the deliberations.

The news jolted South Korea and Japan, where many are deeply skeptical about Mr. Kim’s reported vow to negotiate away his nuclear weapons and fear that Mr. Trump’s “America-first” diplomacy will leave them fending for themselves as China asserts its military prowess.

“For South Korea, living with a nuclear-armed North Korea is much better than living without American troops,” said Shin Won-sik, a retired three-star South Korean general. “If they are gone, we will lose proof that the Americans will defend us. We will lose confidence that if war breaks out, we can win.”

The possibility of a troop reduction emerged as other signs of diplomatic progress on the Korean Peninsula were reported on Friday.

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President Trump visiting American and Korean service members at Camp Humphreys, in South Korea, in November. Mr. Trump has openly questioned the merit of maintaining a costly military presence overseas.CreditDoug Mills/The New York Times

In Washington, Mr. Trump told reporters that a time and place for his meeting with Mr. Kim had been set, and that details would be announced soon. And the United Nations aviation agency, which governs international air routes, said its directors would visit North Korea next week to discuss the possibility of opening routes to South Korea.

In Seoul, President Moon Jae-in moved quickly Friday to calm jitters about an American troop reduction, especially among older conservatives, who consider the United States military’s presence a sacrosanct symbol of national security and are deeply skeptical of Mr. Kim’s intentions. Mr. Moon’s office said news that the White House was considering drawing down troops was “not true at all.”

“The Moon government doesn’t want the focus of public attention to move from the denuclearization of North Korea to the withdrawal of U.S. troops yet, which is such a political hot potato,” said Lee Byong-chul, senior fellow at the Institute for Peace and Cooperation in Seoul. “But if a peace treaty is signed, the U.S. troops are bound to peter out. Much of the reason they are staying here will be gone.”

On Friday, the White House announced that Mr. Moon would meet with Mr. Trump in Washington on May 22 for talks about North Korea and other matters.

Conservatives in South Korea bristle at the possibility of a troop withdrawal, arguing that it would expose their country to potential foes far stronger than North Korea, like China and Japan, which have invaded numerous times over the centuries. South Koreans reacted to Washington’s past efforts to pull out troops with calls for arming the country with nuclear weapons of its own.

Although South Korea’s navy and air force are superior to the North’s, North Korea has a much bigger army, including stockpiles of chemical and nuclear weapons and huge batteries of artillery, rockets and missiles that could hit Seoul, a city of 10 million people.

For decades, the American military has protected South Korea and Japan under its nuclear umbrella, and it shares high-tech military surveillance and conducts annual joint war games preparing for any conflict.

For many in the region, giving up that protection is an unsettling prospect, even if peace comes to the peninsula.

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Supporters of Mr. Trump in Seoul last year. For conservative South Koreans, the American military presence is a sacrosanct symbol of national security.CreditJung Yeon-Je/Agence France-Presse — Getty Images

“The reason foreign investors stay in South Korea, and its stock market doesn’t panic even when China’s military prowess grows and North Korea conducts its nuclear weapons, is because of the U.S. military presence here,” said Mr. Shin, who was the South Korean military’s top operational strategist before he retired in 2015.

“If they shake the alliance for the sake of denuclearizing North Korea,” he added, “we will have an economic crisis before a security crisis.”

Mr. Trump’s reported directive came as Seoul and Washington were negotiating to decide how much more South Korea should pay than the $800 million a year it is currently paying for the 28,500 United States troops here. Mr. Trump wants a bigger burden-sharing from Seoul, which has resisted.

Trump administration officials say a full troop withdrawal is unlikely. They say that rethinking the force’s size and configuration was overdue, and that they want to see if a smaller force can provide adequate security.

The fate of the American troops has become one of the most delicate points of discussion as the two Koreas, the United States and China engage in fast-paced diplomacy over how to end the North’s nuclear weapons program.

South Korean officials say North Korea is not insisting on a pullout of American troops during the latest round of negotiations with Seoul and Washington. They say they want the troops to stay as a regional stabilizer even if a peace treaty is signed.

In past negotiations, North Korean officials told their South Korean and American counterparts that they could support the United States military presence in South Korea if Washington and Pyongyang normalized ties and the troops served a “peacekeeping role” to prevent China from becoming a dominant military power in the region.

A troop withdrawal could send a bad signal to Japan, where about 50,000 American military service members are based, analysts said.

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A welcoming ceremony for Defense Secretary Jim Mattis in Seoul in October. The fate of American troops in South Korea has become one of the most delicate points of discussions as the two Koreas, the United States and China engage in fast-paced diplomacy over how to end the North’s nuclear weapons program.CreditJung Yeon-Je/Agence France-Presse — Getty Images

“Across the waters in Japan, I think it’s going to be read very, very poorly and really make the Japanese anxious about what exactly are the U.S. commitments” in the region, said Jeffrey Hornung, a political scientist at the RAND Corporation.

Under Prime Minister Shinzo Abe, Japan has stepped up its own show of military force, and Mr. Abe has sought to revise the pacifist clause in the country’s Constitution. On Thursday, he released a video message calling for amending it to make explicit the legality of the country’s Self-Defense Forces, as Japan’s military, which has about 225,000 active members, is known.

On Friday Mr. Abe’s office called American forces “essential” for the security of the region.

If Mr. Trump were to succeed in pulling troops from the Korean Peninsula, it could embolden Mr. Abe to push through a constitutional change while citing the reduced American military presence in the region.

The Japanese public has long opposed any constitutional change and on Thursday, thousands of people protested the idea in Tokyo. But recent polls show opinion is increasingly divided.

Ichiro Fujisaki, a former Japanese ambassador to Washington, struck a moderate tone.

He said “the U.S. presence is more symbolic than really there to fight against North Korea,” given the strength of the South Korean military and the fact that North Korea’s missiles are a bigger threat than an actual invasion.

Mr. Fujisaki added that as long as the Trump administration consulted the Japanese and South Korean governments about its plans, “it’s not that big a concern.”

China’s leader, Xi Jinping, spoke Friday with Mr. Moon by phone to offer support for the Koreas’ reconciliation efforts, saying developments were “at an important juncture,” China’s Ministry of Foreign Affairs said on its website. “Peace on the Korean Peninsula is facing a historic opportunity,” Mr. Xi said, according to its summary of the call.

China has long wanted American troops to leave South Korea, and analysts there said a drawdown of United States troops — or a complete withdrawal — could drastically reshape the balance of power in the region.

“The end of American troops in South Korea may indicate the beginning of the dissolution of the U.S. alliance in the Asia-Pacific,” said Cheng Xiaohe, a professor of international relations at Renmin University in Beijing. “If the American alliance system was gone, China would be greatly relieved.”

Choe Sang-Hun reported from Seoul, and Motoko Rich from Tokyo. Reporting was contributed by Jane Perlez and Chris Buckley from Beijing, Hisako Ueno from Tokyo and Rick Gladstone from New York.

A version of this article appears in print on , on Page A6 of the New York edition with the headline: Talk of U.S. Troop Cuts Unnerves Asian Allies. Order Reprints | Today’s Paper | Subscribe